As is known, systems for harvesting energy (also known as “energy harvesting” or “energy scavenging” systems) from intermittent environmental energy sources, have aroused and continue to arouse considerable interest in a wide range of technological fields. Typically, energy harvesting systems are designed to harvest, store, and transfer energy generated by mechanical sources to a generic load of an electrical type.
One of the main energy sources that can be used for harvesting mechanical energy and converting it into electrical energy is constituted by piezoelectric or electromagnetic devices. Low-frequency vibrations, such as for example mechanical vibrations of disturbance in systems with moving parts, can be a valid energy source.
The main needs that are felt in the field of systems for harvesting energy from environmental sources include minimum consumption of energy of the components of the systems themselves, maximum efficiency for harvesting, conversion, and storage of energy, and the need to supply the energy stored to a plurality of devices different from one another that use it for their operation.
FIG. 1 is a schematic illustration, by means of functional blocks, of an energy harvesting system of a known type.
The energy harvesting system 1 of FIG. 1 comprises: a transducer 2, for example of an electromagnetic or piezoelectric type, subjected in use to environmental mechanical vibrations and configured for converting mechanical energy into electrical energy; a scavenging interface 4, for example comprising a rectifier circuit, configured for receiving at input an AC signal generated by the transducer 2 and supplying at output a DC signal for charging a capacitor 5 connected to the output of the scavenging interface 4; and a DC-DC converter 6, connected to the capacitor 5 for receiving at input the electrical energy stored by the capacitor 5 and supplying it to an electrical load 8.
The global efficiency ηTOT of the energy harvesting system 1 is given by Eq. (1) below:ηTOT=ηTRANSD·ηSCAV·ηDCDC  (1)
where: ηTRANSD is the efficiency of the transducer 2, indicating the amount of energy available in the environment that has been effectively converted, by the transducer 2, into electrical energy; ηSCAV is the efficiency of the scavenging interface 4, indicating the energy consumed by the scavenging interface 4 and the factor of impedance decoupling between the transducer and the interface; and ηDCDC is the efficiency of the DC-DC converter 6.
As is known, in order to supply to the load the maximum power available, the impedance of the load should be equal to that of the source. As illustrated in FIG. 2, the transducer 2 can be represented schematically, in this context, as a voltage generator 3 provided with an internal resistance RS of its own. The maximum power PTRANSDMAX that the transducer 2 can supply at output may be defined as:PTRANSDMAX=VTRANSD_EQ2/4RS; if RLOAD=RS  (2)
Where: VTRANSD_EQ is the voltage produced by the equivalent voltage generator; and RLOAD is the equivalent electrical resistance on the output of the transducer 2 (or, likewise, seen at input to the scavenging interface 4), which takes into due consideration the equivalent resistance of the scavenging interface 4, of the DC-DC converter 6, and of the load 8.
On account of the impedance decoupling (RLOAD≠RS), the power at input to the scavenging interface 4 is lower than the maximum power available PTRANSDMAX.
The power PSCAV stored by the capacitor 5 is a fraction of the power recovered by the interface, and is given by Eq. (3):PSCAV=ηTRANSD·ηSCAV·PTRANSDMAX  (3)
whilst the power PEL_LOAD supplied at output by the DC-DC converter to the electrical load 8 is given by the following Eq. (4):PEL_LOAD=PDCDC·ηDCDC  (4)
where PDCDC is the power received at input by the DC-DC converter 8, in this case coinciding with PSCAV.
The main disadvantage of the configuration according to FIG. 1 regards the fact that the maximum voltage supplied at output by the scavenging interface 4 is limited by the input dynamics of the DC-DC converter 8.
The voltage VOUT across the capacitor 5 (supplied at output by the scavenging interface 4 and at input to the DC-DC converter 8) is in fact determined on the basis of the power balancing according to the following Eq. (5):PSTORE=PSCAV−PDCDC  (5)
where PSTORE is the excess power with respect to the power required by the load, recovered by the interface and stored in the capacitor.
In applications where the transducer 2 converts mechanical energy into electrical energy in a discontinuous way (i.e., the power PTRANSDMAX varies significantly in time) and/or the power PEL_LOAD required by the electrical load 8 varies significantly in time, also the voltage VOUT consequently presents a plot that is variable in time.
This causes, for example, a variation of the efficiency factor ηDCDC, which assumes low values at high values of VOUT. The maximum value of VOUT is moreover limited by the range of input voltages allowed by the DC-DC converter. Maximization of the window of values allowed at input by the DC-DC converter 6 requires a specific design of the DC-DC converter; however, also in the latter case, an upper limit of the range of allowable values for VOUT is imposed.
There is a need in the art to provide system and a method for efficiently harvesting environmental energy that will enable the aforesaid problems and disadvantages to be overcome.